VARIETIES OF PASSIVE IMMUNITY 683 



increase and maintain the protection over a longer period of time. This 

 mixed form of immunization has recently received special study by von 

 Behring in immunization experiments against diphtheria, and will be 

 considered in detail in a later section. 



In curative immunization the conditions are somewhat different. 

 During the course of an infectious disease our body-cells are actively 

 engaged in combating the infectious agent, so that reinforcements, in 

 the form of specific antibodies, are indicated and welcomed for the aid 

 they give in overcoming an infection and the relief they afford our hard- 

 pressed protective mechanism. 



For these reasons it may be stated that the more acute the infection, the 

 greater is the indication for introducing an antibody-laden serum. In 

 chronic infections and in some acute infections we may practise active 

 immunization by introducing a vaccine, with the purpose in mind of 

 stimulating dormant cells to produce antibodies; but, as a rule, it is 

 reasonable to assume that in a severe generalized infection our body- 

 cells are doing their utmost to overcome the infection, and extra stimula- 

 tion may be actually harmful. By introducing antibodies produced in 

 some other animal, however, practically no extra strain is thrown upon 

 the body-cells; on the contrary, they may be relieved when the new 

 antibodies overcome the products of infection, and in this manner afford 

 them an opportunity to recover. Unfortunately, this is actually the 

 case in but a few infections, such as diphtheria, tetanus, and cerebro- 

 spinal meningitis, and does not apply equally well to the larger number 

 of bacterial infections due to the pneumococcus, streptococcus, gonococ- 

 cus, and similar infections. The reasons for failure of passive immuniza- 

 tion in the cure of the last-mentioned infections are now being studied 

 and realized, and means are being discovered for overcoming the diffi- 

 culties, so that serum therapy, in a broad sense, is about to play a more 

 important role in the treatment of many bacterial infections. 



VARIETIES OF PASSIVE IMMUNITY 



While, strictly speaking, all antibodies are probably inimical to their 

 antigens, from the practical standpoint of passive immunization three 

 are of primary importance, namely: (1) The antitoxins, (2) the bac- 

 teriolysins, and (3) the bacteriotropins (immune opsonins). The anti- 

 toxins neutralize their toxins; bacteriolysins cause the death of their 

 respective bacteria if suitable complements are present, and bacterio- 

 tropins accomplish the same end by lowering the resistance of the 



